1. Field of the Invention
The present invention relates generally to lighting, and more particularly to a ceramic discharge chamber for a discharge lamp, such as a ceramic metal halide lamp.
2. Description of the Related Art
Discharge lamps produce light by ionizing a filler material such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the filler material are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized filler material and allows the emitted light to pass through it. The filler material, also known as a “dose”, emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
Conventionally, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into desired chamber geometries after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, in a quartz lamp, at temperatures greater than about 950-1000° C., the halide filling reacts with the glass to produce silicates and silicon halide, which results in depletion of the filler constituents. Elevated temperatures also cause sodium to permeate through the quartz wall, which causes depletion of the filler. Both depletions cause color shift over time, which reduces the useful lifetime of the lamp.
Although quartz lamps can be operated below 950° C. for increased lifetime, the quality of the light produced is compromised, because the light properties produced by the lamp depend on the operating temperature of the discharge chamber. The higher the temperature, the better the color rendering, the smaller the color spread lamp to lamp, and the higher the efficacy.
Ceramic discharge chambers were developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the filler material. European Patent Application No. 0 587 238 A1, for example, discloses a high pressure discharge lamp which includes a discharge chamber made of a ceramic such as translucent gastight aluminum oxide. Typically, ceramic discharge chambers are constructed from a number of parts which are extruded or die pressed from a ceramic powder. For example, FIGS. 1a-1e illustrate five parts which are used to construct a ceramic discharge chamber for a metal halide lamp. The two end plugs with a central bore in FIGS. 1b and 1d are fabricated by die pressing a mixture comprising a ceramic powder and an organic binder. The central cylinder (FIG. 1c) and the two legs (FIGS. 1a and 1e) are produced by extruding a ceramic powder/binder mixture through a die. Assembly of the discharge chamber involves the placement and tacking of the legs to the end plugs, and the end plugs into the ends of the central cylinder. This final assembly is then sintered to form four cosintered joints which are bonded by controlled shrinkage of the individual parts.
The conventional ceramic discharge chamber and method of construction depicted in FIGS. 1a-1e, however, have a number of disadvantages. For example, the number of component parts is relatively large and introduces a corresponding number of opportunities for variation and defects. Also, the conventional discharge chamber includes four bonding regions, each of which introduces an opportunity for lamp failure by leakage of the filler material if the bond is formed improperly. Each bonding area also introduces a region of relative weakness, so that even if the bond is formed properly, the bond may break during handling or be damaged enough in handling to induce failure in operation.
Another disadvantage relates to the precision with which the parts can be assembled and the resulting effect on the light quality. It is known that the light quality is dependent to a substantial extent on the voltage across the electrode gap, which in turn is dependent upon the size of the gap. For example, in 70 watt metal halide lamp, a difference in 1 mm in the gap size produces a voltage difference of about 12-15 volts, which significantly affects the light quality. The number of parts shown in FIGS. 1a-1e makes it difficult to consistently achieve a gap size within an acceptable tolerance without significant effort devoted to optimizing the manufacturing process.
It would be desirable, therefore, to have a ceramic discharge chamber for a discharge lamp which could be manufactured precisely to achieve consistently high quality light, while reducing the opportunities for manufacturing defects to occur.